1. Field of the Invention
The present invention relates to circuit board technology.
2. Background Art
A transmission line is a pair of electrical conductors on a circuit board used to carry an electrical signal and corresponding reference signal. The distribution of a magnetic field within and around a transmission line determines the amount of self-inductance of the transmission line and the mutual-inductance of the transmission line to adjacent signal lines. These inductance values are major factors in defining transmission line characteristics such as crosstalk and characteristic impedance. The amplitude and timing integrity of the signals carried by a transmission line are highly dependent on these two characteristics. The magnetic field distribution of a transmission line is a significant factor in preserving signal integrity, and thus is an important factor in the intended function of a transmission line.
In one configuration, a transmission line may include a signal trace and a reference plane. The signal trace carries the forward electrical signal of the transmission line pair, and the reference plane carries the return current related to the electrical signal. The reference plane may be a ground plane or other voltage plane of a circuit board. The signal trace is typically routed over the reference plane. A layer of a dielectric material separates the signal trace and the reference plane. A net magnetic field around such a transmission line is a vectorial sum of a first magnetic field due to the forward current on the signal trace and a second magnetic field due to the return current on the reference plane. When the return current path on the reference plane is very close in distance and size to the forward current path, the first and second magnetic field components cancel out each other at most locations around the transmission line, due to their opposing phases. In such a case, the net magnetic field tends to be mainly concentrated between the signal trace and the reference plane.
The return current on the reference plane tends to have the highest density under the footprint of the signal trace at higher frequencies, because electromagnetic energy flows in the path of least impedance. However, at lower frequencies, the return current on the reference plane tends to spread outside of the footprint of the signal trace, which can result in a significant amount of net magnetic field fringing from the transmission line to adjacent transmission lines, causing interference.
A common technique used to avoid problems with adjacent transmission lines due to fringing magnetic fields is to add space between transmission lines. Another technique is to add shield traces between transmission lines. These techniques, however, reduce routing density for a given surface area of a circuit board, and may therefore require a body size increase for the circuit board. Such increases in body size result in higher circuit board costs. Thus, improved techniques for avoiding problems with adjacent transmission lines due to fringing magnetic fields are desired.
The mutual capacitance between two adjacent transmission lines on a common circuit board is a major factor in determining important characteristics such as crosstalk and differential impedance (where the adjacent transmission lines belong to a differential pair). The mutual capacitance is directly proportional to the relative permittivity (effective dielectric constant) of the dielectric medium that separates the two transmission lines. The mutual capacitance is directly proportional to the area of overlap between the two transmission lines. Furthermore, the mutual capacitance is inversely proportional to the distance between the two transmission lines.
Thus, the mutual capacitance between two signal traces will increase as the routing density increases (e.g., as the spacing between them decreases). For circuit boards with a reference plane beneath and/or above the signal traces, a proximity of the reference plane(s) to the signal traces aids in reducing mutual capacitance. However, manufacturing challenges and impedance control requirements may pose limitations on how close reference plane(s) can be to signal traces.
As a result, there tends to be some amount of mutual capacitance between adjacent signal traces on a substrate. Furthermore, as the logic noise margins of transmission line signals are reduced, a system level crosstalk budget is also reduced. Thus, techniques for reducing the mutual capacitance between adjacent signal traces are desired that do not compromise routing density.